The present invention relates to an image forming apparatus in which a rubbed image or an excessive edge effect can be repressed.
There has been known an image forming apparatus wherein two-component developing agents containing toner powder and carriers are attracted to the circumference of a developing roller by magnetic force, through charging of the developing roller in accordance with an image to be formed, and the developing roller is pressed against a transfer sheet representing an image carrier so that a toner image may be formed on the transfer sheet.
Incidentally, since developing agents adhering to the circumference of a developing roller are attracted to the circumferential surface of the developing roller by magnetic force, the developing agents are sometimes extended from the circumferential surface in the radial direction of the developing roller to be in a shape of a thorn, like iron sand. This is called a brush of developing agents. When the brush of developing agents is formed, it is feared that a toner image (especially, a trailing edge of the image) is scraped off by the tip of the brush, which causes deterioration of image quality such as a rubbed image on a trailing edge of the image. Further, the developing agents held at a position of the developing roller facing a non-image area where no latent image is formed on an image carrier are sucked electrostatically by voltage on the end portion of a latent image on the image carrier, and thereby, developing agents in quantity of more than necessary are attracted to an end portion of the latent image, resulting in a phenomenon wherein an edge of the image is emphasized. In recent years, in particular, there is a strong demand for a color image and an image of high image quality, and factors to deteriorate image quality need to be removed as far as possible.
In view of the problems in prior art stated above, an object of the invention is to provide an image forming apparatus in which a rubbed image or an excessive edge effect can be repressed.
To attain the object mentioned above, an image forming apparatus of the First Structure is represented by an image forming apparatus for conducting development by making two-component developing agents held by a developing roller to touch an image carrier, wherein the following expression holds with respect to relative velocity of the developing roller to the image carrier (Vs/Vp), a distance between the image carrier to the developing roller (Dsd (xcexcm)) and an amount of conveyed developing agents on the developing roller (Dws (mg/cm2)),
1 less than (Vs/Vp)xe2x89xa6(Dsd/Dws)xc3x970.129xe2x80x83xe2x80x83(1) 
An image forming apparatus of the Second Structure is represented by an image forming apparatus for conducting development by making two-component developing agents held by a developing roller to touch an image carrier, wherein at least one or several patch images each having different density are prepared on the image carrier, patch scan data obtained by scanning image patch density in the direction of image advancing for the patch images prepared are detected, and the first image density data within a prescribed range of the central portion of the patch and the second image density data within a prescribed range on the rear end portion of the patch are calculated, and when an edge on the rear end portion on the patch is judged to be an excessive edge effect or to be a rubbed image based on the first image density data within a prescribed range of the central portion of the patch and on the second image density data within a prescribed range on the rear end portion of the patch, the relative velocity of the developing roller to the image carrier (Vs/Vp) is controlled so that, the second image density data within a prescribed range on the rear end portion of the patch may be within a prescribed density range for the first image density data within a prescribed range of the central portion of the patch.
FIG. 1 is a diagram showing the relationship between a distance from an image carrier to a developing roller (Dsd (xcexcm)) and an amount of conveyed developing agents on the developing roller (Dws (mg/cm2)). In virtue of the studies by the inventors of the invention, it has been cleared that the greater a distance between an image carrier and a developing roller, namely a clearance (Dsd) is as shown in FIG. 1, the lower a possibility that an image is scraped off by a brush of developing agents is, because a length of the brush of developing agents is limited, and thereby, image quality is improved. It has further been cleared that, when an amount of conveyed developing agents on the developing roller (Dws) is make to be greater, image quality is lowered because a brush of developing agents tends to be high.
It has been cleared, on the other hand, that when the relative velocity of the developing roller to the image carrier (Vs/Vp) is made to be greater, image quality tends to worsened even when each of the distance between the image carrier and the developing roller (Dsd (xcexcm)) and an amount of conveyed developing agents on the developing roller (Dws) stays to be the same.
The relationship of the foregoing is expressed as follows by the following expression;
((Vs/Vp)xc3x97Dws)/Dsd=axe2x80x83xe2x80x83(1) 
wherein, xe2x80x9caxe2x80x9d is a constant.
The inventors of the invention obtained the constant xe2x80x9caxe2x80x9d through tests.
(Test Conditions)
Two-component developing agents (toner: particle size 5-10 xcexcm+carrier: particle size 40-100 xcexcm,
magnetization amount 30-60 emu/g)
Toner concentration: 4-8%
Charging amount: 10-50 xcexcC/g
Apparent density: 1.5-2.0 g/cm2 
Under the conditions mentioned above, a scratchy rubbed image on the rear end portion was evaluated visually by changing Dws, Dsd and Vs/Vp, and results shown in Table 1 were obtained. In the table, A shows the result that no rubbed image was observed, B shows the result that a rubbed image was observed and C shows none of A and B.
Since it has been cleared from the results of the tests that a=0.129 is a limit for occurrence of a rubbed image, the expression (1) ((Vs/Vp)xc3x97Dws)/Dsd=a can be expressed as follows.
((Vs/Vp)xc3x97Dws)/Dsd=0.129xe2x80x83xe2x80x83(1xe2x80x2) 
Since it is known that an image noise is increased when Vs/Vp is made to be 1 or less, the following expression (2) is obtained from the foregoing and expression (1xe2x80x2).
1 less than (Vs/Vp)xe2x89xa6(Dsd/Dws)xc3x970.129xe2x80x83xe2x80x83(2) 
Namely, as set forth in the image forming apparatus of the First Structure of the present invention, it has been made known that image quality can be kept to be high when the relationship of the expression (2) holds with respect to relative velocity of the developing roller to the image carrier (Vs/Vp), a distance between the image carrier to the developing roller (Dsd (xcexcm)) and an amount of conveyed developing agents on the developing roller (Dws (mg/cm2)).
In particular, a rubbed image caused by a brush of developing agents appears clearly on a halftone portion of an image. Therefore, it is extremely effective to make the relationship of the expression (2) to hold for keeping image quality in the case of forming an image on a high image quality mode, when an image forming apparatus has a high image quality mode for developing a highly detailed image such as a photograph.
Further, an amount of conveyed developing agents varies depending on a distance for the developing roller to slide. Therefore, the amount of conveyed developing agents can be controlled, without obtaining directly the amount of conveyed developing agents, if the distance for the developing roller to slide is measured integrally and the amount of conveyed developing agents (Dws) is determined based on the sliding distance measured integrally. Incidentally, if the relationship between a sliding distance of the developing roller and the amount of conveyed developing agents is obtained on a test basis, for example, to be stored in a non-volatile memory as a table, it is possible to obtain an amount of conveyed developing agents immediately from the table, if the sliding distance at the present time is known.
Further, an amount of conveyed developing agents varies depending also on a change of ambient temperature and humidity. Therefore, the amount of conveyed developing agents can be controlled, without obtaining directly the amount of conveyed developing agents, if at least one of the ambient temperature and humidity is detected and the amount of conveyed developing agents (Dws) is determined based on at least one of the detected ambient temperature and humidity Incidentally, if the relationship between an ambient temperature and humidity and the amount of conveyed developing agents is obtained on a test basis, for example, to be stored in a non-volatile memory as a table, it is possible to obtain an amount of conveyed developing agents immediately from the table, if the ambient temperature or humidity at the present time is known.
An image forming apparatus of the Second Structure is represented by an image forming apparatus for conducting development by making two-component developing agents held by a developing roller to touch an image carrier, wherein at least one or several patch images each having different density are prepared on the image carrier, patch scan data obtained by scanning image patch density in the direction of image advancing for the patch images prepared are detected, and the first image density data within a prescribed range of the central portion of the patch and the second image density data within a prescribed range on the rear end portion of the patch are calculated from the patch scan data, and when an edge on the rear end portion on the patch is judged to be emphasized or to be scratchy based on the first image density data within a prescribed range of the central portion of the patch and on the second image density data within a prescribed range on the rear end portion of the patch, the relative velocity of the developing roller to the image carrier (Vs/Vp) is controlled so that, the second image density data within a prescribed range on the rear end portion of the patch may be within a prescribed density range for the first image density data within a prescribed range of the central portion of the patch, and thereby, image quality can be adjusted automatically, which is an advantage that no time is taken for adjustment.
It is further possible to conduct control of developing AC frequency or developing AC bias in place of the control of the relative velocity of the developing roller to the image carrier (Vs/Vp).
It is preferable that image density data within a prescribed range on a central portion of the patch are detected as third image density data after the aforesaid control, and feed back control is conducted for developing DC bias or an exposure value.